Transient spectroscopy is used to study the kinetics of conformational changes in macromolecules subsequent to excitation with a pulsed laser. Changes in both the tertiary and quaternary structure of hemoglobin have been observed following the photodissociation of carbon monoxide from the hemes. Steady state photodissociation of carbon monoxide from hemoglobin S is used to study the thermodynamics and nucleation- controlled kinetics of the assembly of deoxyhemoglobin S into polymers. This techniques has been used to study hemoglobin S in partially saturated solutions and to obtain delay times for solutions under physiological buffer conditions. Moreover, the kinetics of polymers formation can be monitored as the cell is being desaturated, permitting, for the first time, determination of the distribution of times required for cells to sickle at the saturations comparable to those of venous blood.